President Donald Trump’s first year in office has been plagued by parallel Russia investigations – one led by special counsel Robert Mueller and the other led by the House Intelligence Committee.

Behind both of these investigations stands British spy Christopher Steele, and the dossier he authored during the 2016 election about Trump’s ties to Russia.

The elusive MI6 agent has a fascinating life, from his time uncovering political assassinations to his current role as a defining figure in the Russia probes.

The life of Christopher Steele reads like the script of an international spy thriller.

From the shores of Yemen to MI6 offices in Moscow and London, the British spy amassed an impressive career before he compiled the infamous yet seminal dossier about then-US presidential candidate Donald Trump’s ties to Russia in 2016.

The elusive agent’s credibility and expertise have come under increasing scrutiny over the last several months.

While the explosive claims about Trump’s collusion with Russia that Steele makes in his dossier have yet to be independently corroborated, little by little, investigators are confirming bits and pieces of the document. Now, a second dossier has emerged that reportedly came to many of the same conclusions Steele did.

Although he has been maligned as a political operative who compiled the dossier for partisan gain, it is clear from his activities that over time, the dossier became a deeply personal matter for Steele. He saw its completion as a matter of national security for both his native United Kingdom and the United States.

Here’s a look at how Steele became a vital intelligence source on both sides of the Atlantic – and a defining figure in the Russia investigation in the process:

Steele’s story begins in an unlikely location — on a colonial army base in the port of Aden, Yemen, on the Arabian Sea, where he was born in 1964.

In addition to Yemen, he spent time in Cyprus and in the Shetland Islands of Scotland, where he developed a passion for bird-watching.

After attending two different boarding school in England, Steele was admitted to Cambridge University, where he became the president of the famed Cambridge Union debate society. He graduated in 1986 with a degree in Social and Political Sciences.

Upon graduation, Steele was immediately recruited by MI6, the British Secret Intelligence Service. After working in London for several years, in 1990 at age 26, he was assigned to his first foreign posting — at the British Embassy in Moscow.

While in Moscow, Steele worked undercover and posed as a British diplomat. He was an “internal traveller” who was able to visit parts of Russia previously inaccessible to foreigners.

Steele was witnessing a nation in transition. He was in Moscow the day the Soviet Union fell. But as the KGB agents keeping tabs on him became rebranded as agents of the Federal Security Service (FSB), he realised the old system had remained firmly in place.

Steele left Moscow in 1993, and after a stint in Paris, was sent to Bagram Airfield in Afghanistan in 2003 to train members of the Special Forces on “kill or capture” missions against the Taliban.

By 2004, Steele was at the top of his game. He served as a senior officer within MI6, and his expertise on Russia was described as “superb.” In 2006, he became the official head of the Russia desk at the agency.

Oli Scarff / GettyA general view of the headquarters of the British Secret Intelligence Service, also known as MI6, in Vauxhall, London.

In 2006, Steele became the case officer looking into the death of ex-FSB agent Alexander Litvinenko, who had defected to Britain. It was Steele himself who eventually determined that the Russian state poisoned Litvinenko.

In 2009, tragedy struck for Steele — his wife Laura died of liver disease. He left MI6, and founded his own private intelligence company that did work on Russia and Ukraine. In 2010, his firm Orbis Business Intelligence helped launch the investigation into FIFA’s global corruption.

Years passed. Steele’s reputation continued grow, and Orbis authored hundreds of reports on Russia and Ukraine that both US and UK officials trusted. An intelligence expert said he was the “go-to person on Russia in the commercial sector.”

In spring 2016, Glenn Simpson, the founder of research firm Fusion GPS, approached Steele with an interesting proposal. He enlisted Steele to begin gathering intelligence on then-candidate Donald Trump — all for an anonymous client.

In 2015, the conservative publication “The Washington Free Beacon” had hired Fusion to conduct opposition research on Trump. Once the Beacon dropped the investigation in spring 2016, the Democratic National Committee picked up funding where they left off.

This was where Steele stepped in. Reportedly, Steele didn’t know the Democratic Party was financing his research, and was only told to focus on Trump’s campaign chairman, Paul Manafort. But his investigation soon broadened to include Trump himself.

Over the course of the next 6 months or so, Steele compiled a series of 17 reports that together would come to be known as the Trump-Russia dossier.

The first thing Steele says he learned was that Russians had compromising information on an alleged escapade Trump had with prostitutes in Moscow. Even at this early stage, Steele was rattled by his findings, and reportedly told colleagues he was “sitting on a nuclear weapon.”

In July 2016, he met with an FBI contact he knew in London to go over his findings. At this point, Steele was still working solo, but the meeting set the groundwork for future cooperation with the intelligence agency.

Steele moved forward with his investigation, and uncovered even more stunning information. Throughout July and August, he found evidence of a massive Russian hacking operation, and indications the Kremlin was pleased with its efforts to divide the US.

Over the next few months, Steele found something that seemed unfathomable — that Moscow had secretly been grooming Trump for the past five years, and that he was complicit in Russia’s efforts to swing the upcoming 2016 election in his favour.

The two spent hours poring over his source material in Steele’s home near London, and determined his sources would have had little incentive to lie to him.

In October 2016, Steele formally met with FBI agents in Rome for questioning. According to Fusion GPS co-founder Simpson, Steele told him that “If there’s a national security emergency or possible national security issue, I should report it.”

Following his questioning by the FBI, Steele agreed to be an informant for them. But Steele’s private intelligence gathering now seemed to be at odds with his sense of public duty, which required him to cooperate with the US government.

Sen. Lindsay Graham, who is well connected in the intelligence community, reportedly offered Steele this bit of advice at the time: “You can be an FBI informant. You can be a political operative. But you can’t be both, particularly at the same time.”

But Steele grew frustrated with US intelligence agencies, and felt their investigations were not moving as quickly as they should be.

“We were very concerned that the information that we had about the Russians trying to interfere in the election was going to be covered up,” Simpson said on the FBI’s relative inaction on Steele’s tips during Congressional testimony.

So in September 2016, Steele decided to go rogue. He and Simpson held off-the-record meetings about his findings with several news publications, including the Washington Post, the New York Times, and CNN.

On October 31, 2016, Mother Jones became the first publication to mention Steele’s dossier, and published an article titled, “A Veteran Spy Has Given the FBI Information Alleging a Russian Operation to Cultivate Donald Trump.”

Even though Mother Jones hadn’t mentioned Steele in their article by name, the FBI promptly ended their relationship with him. But by this time, Steele’s claims had begun circulating in the Obama White House.

Then, Trump won the November election. With the end of Trump’s campaign, Steele’s funding from the Democratic Party ended, too. He decided to keep working on his dossier though — even without pay. He filed the dossier’s final report in December.

Mark Wilson/Getty ImagesRepublican president-elect Donald Trump delivers his acceptance speech during his election night event at the New York Hilton Midtown in the early morning hours of November 9, 2016 in New York City.

Steele and his allies knew it was imperative US lawmakers knew about his research. Through a series of clandestine handoffs, Steele passed the dossier to Sen. John McCain, who, after reviewing it, reportedly gave it to former FBI director James Comey.

At this point, the dossier had become Washington’s worst kept secret. Because it was not independently verified, news outlets shied away from it — until Buzzfeed became the first publication to print it in its entirety on January 10, 2017.

Of course, Trump and Russian President Vladimir Putin denied its many unverified allegations. And as the world grappled with the dossier’s implications for the first time, Steele became lauded by some — and derided by others.

Steele’s dossier became the basis for the controversial memo on misconduct at the FBI and Justice Department authored by Rep. Devin Nunes, and Sen. Graham and Sen. Chuck Grassley referred him for a criminal investigation because of his work with the FBI.

Meanwhile, Steele himself was living quietly. According to news reports, he visited Washington to testify before special counsel Robert Mueller’s team in the ongoing Russia probe in September, but that’s about all we know of his whereabouts since early 2017.

Alex Wong/GettyMueller confirmed that the FBI uses drones for domestic surveillance during a hearing on FBI oversight in 2013.